1. Field of the Invention
The present invention relates to a protein, mutants thereof, and nucleic acids encoding said protein, that interacts with Ice1 and which activates CBF3 promoter activity thus regulating freezing tolerance in plants.
2. Discussion of the Background
Adverse low temperature affects the survivability and distribution of almost all living organisms. Plants being sessile have evolved to sense and encounter low temperature stress and their response to adverse low temperature is manifested at physiological, molecular and biochemical levels. Many temperate plants have potential to increase freezing tolerance of these plants after a prior exposure to non freezing temperatures—a process known as cold acclimation (Guy 1990; Hughes and Dunn 1996; Browse and Xin 2001).
At a molecular level a specific set of proteins are induced in response to low temperature that helps plants to sustain adverse low temperature conditions. Moreover, gene expression has been observed to change in response to cold, which is critical for chilling tolerance (Gong et al. 2002; Hsieh et al. 2002) and cold acclimation (Thomashow 1999; Knight et al. 1999; Tahtiharju and Palva 2001). Proteins induced during cold acclimation include enzymes involved in respiration and metabolism of carbohydrates, lipids, phenylpropanoids and antioxidants; molecular chaperones, antifreeze proteins; and many others with presumed function in tolerance to dehydration caused by freezing (Thomashow 1999; Guy 1990; Mohapatra et al 1989). These genes and gene products have been termed CAPs (cold acclimation proteins)/CORs (cold responsive/LTIs (low temperature inducible).
Promoters of many of these genes have DRE/CRT (dehydration responsive element/C-repeat), a cis element necessary and sufficient for gene transcription under cold stress (Yamaguchi-Shinozaki and Shinozaki 1994; Stockinger et al 1997). A small group of homologous transcription factors (CBF/DREB) bind to this sequence and induce cold-regulated gene expression (Stockinger et al 1997; Liu et al 1998). Recently the present inventors have identified an upstream factor that binds to the Myc like sequences of the CBF3 promoter and is a critical determinant of CBF3 expression and freezing tolerance in Arabidopsis (Chinnusamy et al 2003). Apart from the Myc like sequences in CBF3 promoter there are many putative Myb like sequences (Shinwari et al 1998). MYC-related bHLH transcription factors require MYB co-transcription factors and/or WD-repeat containing factors for transcriptional activation of target genes (Spelt et al. 2000; Walker et al. 1999).
Chinnusamy et al (2003) proposed that possibly a Myb like transcription factor interacts with Ice1 and causes cold-induced expression of CBF genes. Importantly, microarray analysis show more levels of Snow1 (previously referred to as AtMyb15 in U.S. 60/508,316 to which the present application claims priority and which is incorporated herein by reference in its entirety) transcript in ice1 mutant when compared to wild type plants under conditions of cold stress (Chinnusamy et al 2003). This indicates that in absence of Ice1 function, more levels of Snow1 may be produced to compensate the loss of function of Ice1.
Since environmental factors, such as cold, limits the geographical distribution and growing season of many plant species, and often adversely affects crop quality and productivity, there remains an ongoing critical need to increase cold tolerance and/or cold acclimation in plants, particularly those plants that are advantageously useful as agricultural crops.